The technical field of this invention is CMOS imaging devices having high dynamic range.
Charge-coupled devices (CCDs) have been the basis for solid-state imagers for many years. However, during the past ten years, interest in CMOS imagers has increased significantly. CMOS imagers are capable of offering System-on-Chip (SOC) functionality, lower cost, and lower power consumption than CCDs.
Dynamic range is one of the most important performance criteria of image sensors. It can be defined as the ratio of the highest illuminance to the lowest illuminance that an imager can measure and process with acceptable output quality. The dynamic range of conventional CMOS imagers is usually limited to about 60 dB because the photodiode has a nearly linear response and all pixels have the same exposure time. However, many scenes can have a dynamic range of more than 100 dB.
One way to improve dynamic range is to use the logarithmic response of a MOS transistor in the sub-threshold region. However, the signal-to-noise ratio (SNR) with this method is poor. Another method is to adjust the photodiode""s well capacity. This technique can achieve high dynamic range of 96 dB. However, the SNR of this scheme is also poor. Multiple sampling is another method of increasing the dynamic range of imagers. Due to the linear nature of multiple sampling, either ultra-fast readout circuits or very long exposure times are required. In order to achieve a dynamic range of about 120 dB, the imager needs exposure times as long as six seconds. Obviously, this is not feasible for video mode operation. Furthermore, multiple sampling requires each pixel""s values to be read off multiple times in one frame. This can lead to high power consumption.
Researchers recently reported a CMOS imager that achieves a dynamic range of 120dB by representing illuminance in the time domain. Pixels output pulse events, with a brighter pixel outputting more pulse events than a darker pixel. This process is continuous because, in contrast with conventional imagers, there is no global imager reset that signifies the start of a frame. When a pixel emits a pulse event, the imager outputs that pixel""s address using address event readout circuits. The scene is reconstructed by measuring how frequently the pixels output pulse events. Therefore, the times of the past pulse events of the pixels must be stored in a large memory. Additionally, bright pixels may output many pulse events over a short period of time, so the load on the arbiter may become large.
An image sensor includes a plurality of pixel cells disposed in a two dimensional array. Each pixel cell includes a photodiode and a comparator circuit. The comparator compares the photodiode voltage with a common reference voltage. Upon a match the comparator circuit generates a comparator pulse supplied to a corresponding row line. A row arbiter detects when a row line receives a comparator pulse from a pixel in said corresponding row. This row arbiter generates a row signal indicative of the row, selecting a single row if more than one row line receives a comparator pulse simultaneously. The row arbiter enables the comparators of pixels in the selected row to send the comparator pulses to corresponding column lines. A column arbiter detects when a column line receives a comparator pulse from a pixel in the corresponding column and generates a column signal indicative of the column. The column arbiter selects a single column if more than one column line receives a comparator pulse simultaneously. The time of arrival of a comparator pulse indicates the illumination level of the corresponding pixel.
The image sensor preferably includes a row encoder and a column encoder. The row encoder receives the row signal and generates a multi-bit digital row signal indicative of the row. The column encoder receives the column signal and generates a multi-bit digital column signal indicative of the column.
The optimum contour of the common reference voltage depends upon the particular application. The common reference voltage is a voltage ramp in the simplest case. The reference voltage monotonically changes from a voltage indicative of maximum illumination at a time corresponding to a minimum exposure time to a voltage just above a signal to noise ratio at a time corresponding to a maximum exposure time.